1. Field of the Invention
This invention relates to improved two phase anaerobic digestion of organic carbonaceous materials wherein a large portion of the carbon dioxide contaminant is removed from the acid forming phase and from the liquid product of the acid forming phase resulting in high methane content product gas recovery from the methane forming phase. The high methane content product gas is further enhanced by carbon dioxide absorption into the liquid phase during the methane forming phase of the anaerobic digestion which is conducted under pressurization.
2. Description of the Prior Art
Anaerobic digestion is a biological process which has been used by industry and municipalities for many years to facilitate sludge stabilization and pollution control efforts. During the anaerobic digestion of organic material, product gas containing mainly methane and carbon dioxide is released. As world energy requirements have increased and as energy resources have become increasingly expensive and unreliable, interest in developing alternative efficient and economically feasible energy resources has heightened. The anaerobic digestion process effects the bioconversion of various kinds of land and water bbiomass and various wastes to product gas containing methane. Anaerobic processes can be used to convert inexpensive, readily available, and inexhaustible resources, notably sludge, industrial and municipal waste and biomass material, to valuable product gases, principally methane. Utilization of such processes is presently limited, largely because expensive and energy-intensive processing is required to convert the raw product gas to a usable gas having a sufficiently high energy value.
The composition of raw product gas generated from single phase anaerobic digesters is typically 55 to 65 percent methane, 35 to 45 percent carbon dioxide, and less than 1 percent hydrogen and hydrogen sulfide. This raw product gas must undergo additional processing to provide gas containing greater than 90 percent methane as preferred for utilization as pipeline quality gas.
Two phase anaerobic digestion processes have been developed in which an acid forming anaerobic digestion is conducted first. In the acid phase, the microbial population and operating conditions are selected to promote the conversion of organic carbonaceous matter to volatile fatty acids of low molecular weight. The volatile fatty acids remain solubilized in the liquid effluent of the first stage, and are conveyed to a second anaerobic digester where a methane producing digestion occurs. In the methane phase, methanogenic microorganisms convert the volatile fatty acids to product gas comprising primarily methane and carbon dioxide. This two phase anaerobic digestion process significantly increases the methane content of the product gas. Product gas released from the second phase may contain up to about 75 percent methane which still requires further processing for utilization as high quality pipeline gas.
Several publications focus on various aspects of the anaerobic digestion process with the objective of increasing the conversion efficiency or the methane content of product gas. U.S. Pat. No. 4,022,665 teaches a two phase anaerobic digestion which utilizes a short retention time of less than two days in the acid forming phase and a longer, 2 to 7 day retention time in the methane producing phase. The U.S. Pat. No. 4,022,665 teaches methanation of carbon dioxide in the methane production phase. A two phase anaerobic digestion system is described in E. Colleran, "The Application of the Anaerobic Filter Design to Biogas Production from Solid and Liquid Agricultural Wastes", Proceedings of the Symposium on Energy Biomass and Wastes, Institute of Gas Technology, 1982, wherein limestone chips are used in a packed bed methane phase digester to result in about 90 percent methane content product gas. U.S. Pat. No. 3,981,800 teaches a process wherein combined phase (acid formation and methanogenesis performed in the same digester) anaerobic digestion is conducted under several atmospheres pressure and carbon dioxide is stripped by depressurization resulting in 98 percent methane containing product gas. Two combined phase digesters may be used in series with both acid forming and methanogenic digestion in each. Maintenance of specific volume to interface ratios is taught by U.S. Pat. No. 4,040,953 to allow preferential diffusion of methane while carbon dioxide is retained in the slurry. A photo-culture of algae is employed by the process of U.S. Pat. No. 4,354,936 to remove carbon dioxide from the product gas by absorption. U.S. Pat. No. 4,396,402 discloses a method by which organic materials undergo a process of accelerated bioleaching before being introduced as substrate in a two phase anaerobic digestion process. U.S. Pat. No. 4,352,738 teaches a combined phase recycling anaerobic filter for aqueous organic wastes and adjustment of chemical oxygen demand to result in 80 to 90 percent methane; U.S. Pat. No. 4,315,823 teaches a combined phase recycling anaerobic filter for acidic petrochemical wastes operated at relatively high carbon dioxide partial pressure; and U.S. Pat. Nos. 4,351,729, 4,366,059 and 4,349,435, all relate more generally to anaerobic filters for treatment of liquids.